In air-tight storage structures for storing silage or the like, a bottom unloader is employed having a rotating cutter arm which undercuts the silage and conveys the silage to the inner end of a trough formed in the foundation of the silo. A discharge conveyor operating within the trough then conveys the silage to the exterior of the silo.
In a bottom unloader, as disclosed in the U.S. Pat. No. 2,635,770, the cutter arm includes an endless cutter chain that is engaged with a drive sprocket located at the inner end of the arm and with an idler sprocket mounted on the outer end of the cutter arm. Spaced along the length of the cutter chain are a series of knives that cut and dislodge the silage and convey the silage to the inner end of the trough as the chain travels in its endless path.
The cutter chain travels in a horizontal plane and as the knives or teeth on the cutter chain travel in close proximity to the foundation of the storage structure, sag cannot be tolerated in the chain so that a zero catenary chain system is utilized.
Chains which operate with zero catenary require special attention, due to the forces generated in the system by virtue of pre-tension on the chain. With a zero catenary system the chain bushings are biased inwardly against the roots of the sprocket teeth by the tensioning force and the links cannot move outwardly along the sprocket tooth as the links elongate due to wear. This is due to the fact that the pre-tensioning in the system exceeds the ejection force so that the chain roller or bushing is maintained on the sprocket root. The pre-tensioning thus forces the bushing to move along the root of the sprocket tooth to compensate for its increase in pitch length. As the elongation of the link continues, the bushing will contact the junction between the root and the non-driving tooth face of the next succeeding sprocket tooth, causing erosion of this area of the sprocket, as well as the bushing, and resulting in premature failure of the chain and sprocket.